Spray corrosion testers are extensively employed to test the corrosive property of coatings such as paint or rust-proof oil films, which coatings may be applied to metal surfaces such as aluminum, iron and stainless steel plates and the like.
In the conventional spray tester, a corrosive aqueous solution containing about 5% salt water and, if necessary, cupric salts, are sprayed through a nozzle over a test piece placed on a suitable holder provided within the tester tank. The temperature and relative humidity within the tank is maintained at a constant level. The extent of rust generation on the surface of the test piece is then examined.
The test apparatus is, of course, assembled from corrosion-resistant materials.
The results of the tests, however, are greatly affected by the nature of the spray in the test apparatus, atmospheric conditions and the construction of the test apparatus itself.
Typically, the following test conditions are required.
1. The temperature ranges from 35.degree. to 50.degree. C, and tolerance should be within .+-.1.degree. C.
2. The amount of spray received on the holder carrying the test piece should be between 1 and 2 cc/80cm.sup.2 per one hour and be uniformly distributed.
3. The salt concentration in the spray varies since salt concentration in the solution within the salt water tank increases with lapse of time. Moreover, the spray itself is quite concentrated and periodic inspection is required to insure uniformity of sale concentration in view of the foregoing considerations.
4. In order that drops of the solution which adhere to the inner surface of the test tank lid not directly fall on the test piece, the lid must be tilted.
Further, the lid of the tester tank is required to have a structure capable of being quickly opened and closed to observe the test piece during or after the test. Also in order to prevent leakage of the spray, the lid must form a perfect seal with the tank.
A typical seal of the prior art is shown in FIG. 1, wherein the lower edge of lid 2 is immersed in a water pool provided in a groove provided around the circumference of the tester tank.
This prior art apparatus, however, has the following drawbacks:
1. In the water seal around the circumference of the tank, a gap is naturally formed between the tank and the lower edge of the lid by a rubber gasket provided as a buffer between the lid and the groove, thereby permitting water to flow between the outer and inner sides of the lid, so that heated water from the inner side becomes admixed with the water in the outer side. Consequently, heat from the tank is lost and the temperature in the area of the seal is between atmospheric temperature and the temperature of the air within the bulk of the tank. This temperature differential within the tank is obviously undesirable.
In a typical case, where atmospheric temperature is 25.degree. C and the temperature inside the tank is 35.degree. C, the temperature difference within the tank may be as high as 5.degree. C unless the flow of water is stopped.
2. In view of the foregoing, with a conventional water seal, the lid and the adjacent wall area are cooled, resulting in the hydroextraction of the atmosphere in the area around the lid and adjacent wall areas inside the tank, thereby increasing the salt concentration of the spray.
3. With the cooling of the lower side of the lid, the surrounding atmosphere is disturbed and this increases the descending air stream in comparison with the flow of air in the center of the tank. This results in a loss of uniformity of spray distribution at the outer portions of the tank, in comparison with the inner portions. Simultaneously, the temperature differential of the test piece holder is increased.
It will be seen from the foregoing, that there is a close relationship between the above-discussed factors and, consequently, it is difficult to control said factors and this often results in great fluctuations in spray test data.